Positive Fluid Displacement Devices (PFDD's) are used extensively to meter and/or transfer fluids, either gas or liquids, in the medical and Industrial fields. Types of PFDD's include rotary (gear, vane), piston, syringe, diaphragm and peristaltic. All of these PFDD's have drawbacks which reduce their accuracy and efficiency, for example, gear, vane and piston devices generally have internal leakage problems which result in requiring high-speed operation to minimize volumetric losses.
Some types of PFDD's have a significant dead volume in the displacement chamber due to manufacturing tolerances, valves, seals etc., thus reducing the pressure ratio and accuracy, particularly critical in the transfer of gas, and making it difficult to eliminate air bubbles in the transfer of liquid.
Certain designs require lubricating oil to seal the displacement chambers of PFDD's, resulting in contamination of the transferred fluid (air compressors, vacuum pumps).
So called "dry lubricated" PFDD's often utilize Teflon.RTM. coated or Teflon-derived materials for moving wetted parts. Such devices are subject to the generation of particles during the wear process thereby contaminating the fluid.
Other designs of PFDD's with fixed clearances between moving wetted parts exhibit decreasing performance as wear takes place. Such devices often allow particles contained in transferred fluids such as slurries to be trapped between moving wetted parts, resulting in jamming or premature wear (vane, gear, piston).
Some PFDD's may damage the fluid, particularly liquids containing non-homogeneous material such as blood, due to large sealing surfaces creating shearing action to the liquid caught between the sealing surfaces (gear, vane, peristaltic).
In many PFDD's, it is difficult and often impossible to change the wetted parts to handle aggressive or dangerous fluids such as acids, bleaches, toxic, and abrasive substances without changing the entire device. Such devices generally do not satisfy the requirements of the chemical and medical fields in an economical fashion.
Some PFDD's incorporate abrupt changes of direction and lengthy restricted and/or convoluted internal fluid passageways thereby creating turbulent fluid flow and cavitation resulting in loss of accuracy and performance. Such design often renders it difficult if not impossible to flush and/or backwash the PFDD for cleaning purposes. Such devices are not suitable for metering the delivery or drawing of precise fluid quantities.
Many PFDD's lack precision in the volume delivered or drawn. This is especially the case with devices which have uncontrolled compliance in the pumping chambers (diaphragm, peristaltic).
Some PFDD's require valves and are not reversible. Such devices (piston, diaphragm) cannot be used to deliver or draw fluids by simple reversal of the driving motor. They are also unsuitable as flow meters.
Many PFDD's have a limited range of operation and lack low speed capability because accuracy of fluid displacement is lost when the volume of fluid displaced is reduced from rated capacity to near zero.
It is an object of this invention to provide a positive fluid displacement device which solves the many problems of prior art devices outlined above. This invention is based on the travelling cylinder concept described in U.S. Pat. No. 5,004,404, which is related to U.S. Pat. No. 4,907,950. In the travelling cylinder concept two opposing pistons are connected to each of two eccentrics or crankpins on a crankshaft. The opposing displacement chambers are secured together as one piece and are also connected to the crankshaft. Although the chambers are anchored to the crankshaft, they are permitted to move laterally in a direction perpendicular to the sliding direction of the piston inside the chamber. The outer end of each displacement chamber is provided with intake and exhaust openings. These openings mate with openings in the housing which encloses the device. The housing has exhaust channels to carry out the fluid delivered by the device. The intake port openings in the housing connect internally so that the incoming fluid is forced to flow around the internal moving parts for cooling purposes before entering the displacement chambers. The device described in U.S. Pat. No. 5,004,404 was developed for the pumping of air and was specifically oriented toward a device for supercharging internal combustion engines. A mechanism was provided to vary the displacement of the cylinders by changing the stroke of the pistons. The design of the prior art device was not capable of meeting all of the problems outlined above. In particular, it displayed some amount of leakage between the moving cylinder and the stationary ports in the housing; also, it was not designed for pumping liquids in precise, metered amounts.